This invention relates to a battery separator for use in a gas recombinant type lead acid battery and to a method of making the separator.
In lead acid batteries the positive and negative electrodes or "plates" are separated by a battery separator. In recombinant type lead acid batteries the separator is a porous mat wettable by the electrolyte. Substantially all of the electrolyte in a recombinant type lead acid battery is contained in the separator.
The separator of choice in recombinant batteries currently produced is a glass fiber mat. Typically such glass fiber mats have a basis weight of between about 140 and about 170 grams/square meter. Glass fiber separators do not have good puncture resistance and cannot be formed into an envelope around a plate.
U.S. Pat. No. 5,009,971 discloses a gas recombinant separator that is a non-woven, porous flexible sheet consisting essentially of a substantially homogeneous mixture of from 93 to 99.5 weight percent amorphous silica and from 0.5 to 7 weight percent of fibrillated, unsintered polymeric material, preferably polytetrafluoroethylene ("PTFE"). However, the tensile strength of this separator sheet is too low, less than 75 pounds per square inch, and its bulk density is too high to make it an economical alternative to glass mat.
U.S. Pat. No. 4,153,661 discloses a method of making PTFE composite sheet material. The method comprises dry blending a particulate material (such as silica) and PTFE particles in a weight ratio of 3:1 to 50:1, adding sufficient lubricant (water) to the blend to exceed the absorptive capacity of the solids yet maintaining a putty-like consistency, mixing the putty-like mass in an intensive mixer at a temperature between 50 and 100 degrees C. for a time sufficient to cause fibrillation of the PTFE particles, biaxially calendering the putty-like mass to cause additional fibrillation and to form a self supporting sheet, and drying the sheet to remove the water. This patent discloses several uses for the sheet, including use as battery separators.
U.S. Pat. No. 5,071,610 discloses an improvement to the process (such as disclosed in U.S. Pat. No. 4,153,661) of making a sheet comprised of a PTFE fibril matrix and insoluble, non-swellable sorptive particles enmeshed in said matrix. The improvement involves using a lubricant (such as water, alcohols or other organic solvents) in an amount to exceed the absorptive capacity of the particles by at least 3 weight percent, the ratio of insoluble particles to PTFE being in the range of 40:1 to 1:4. The sheet is said to be useful for analytical and preparative purposes in separation science, and in analysis in the fields of clinical, biological and environmental sciences.
It is an object of this invention to provide a separator for a recombinant lead acid battery comprising a sheet formed of a PTFE fibril matrix and a particulate filler enmeshed in said matrix that has high tensile strength, low bulk density and high void volume.
It is a further object of this invention to provide a process for making such a separator.